DESCRIPTION (Applicant's Abstract): Microfibril-associated glycoprotein-2 (MAGP-2) is one of two MAGPs, both of which are integral components of microfibrils associated with elastic fibers. MAGP-2 is shown herein to bind specifically to fibrillin-1 and -2, two well-characterized constituents of extracellular microfibrils that are genetically linked to heritable disorders of elastic fibers known as the Marfan syndrome and congenital contractural arachnodactyly, respectively. MAGP-2 also binds MAGP- 1. In this proposal, the function of MAGP-2 within the microfibril will be examined. MAGP-2 binding sites on both fibrillin-1 and MAGP-1 will be localized, as will determinants within MAGP-2 responsible for these. The spatial and temporal pattern of MAGP-2 gene expression in the developing mouse will be determined, and coexpression/colocalization with other major elastic fiber components will be assessed. MAGP-2 expression in the MAGP-1 knockout mouse will also be investigated. A mouse harboring a targeted disruption of the MAGP-2 gene will be generated and assessed for developmental abnormalities. Microfibrils in MAGP-2 -/- mice will be studied at the ultrastructural level. If the mouse dies during embryonic development, primary fibroblast cultures will be derived from MAGP-2 -/- lungs and used to study the role of MAGP-2 in microfibril assembly in vitro. The role of MAGP-2 in lung branching morphogenesis will also be studied in knockouts should their lungs develop abnormally. If the MAGP-2 -/- mice lack an overt phenotype, they will be bred with MAGP-1 knockout mice to make mice lacking both MAGPs to assess whether the MAGPs have overlapping yet critical functions.